IIRC the 80's injected Chargers and maybe others used a speed sensor that connected to the trans and then the speedo cable connected to the sensor so you could have both. I don't know if they used the same counter though. But I think there are only 4 poles in the sensor. If you keep the speed sensor and brake light switch signals you won't have to use switches for the O/D and lockup.
I'm going to swap a complete 93 Ram Charger dash, engine, HVAC, fuel tank and wiring harnesses into my 74 crew cab. I plan to use everything from the RC including the steering column so mine should be easy. What I would like to do though is an OBD II swap for the engine management so I can modify the engine with an intake, cam and heads. The hot wire harness looks like an easy way to go but at $1000 it's pretty steep. Especially when you consider you still have to get a PCM and tune it as well. So it may remain stock for awhile.
The Scamp is sold, now just two old trucks.
'92 D250 Club Cab CTD, 47RH conversion, pump tweaks, injectors, rear disc conversion. Hydroboost coming soon
'74 W200 Crew Cab 360, NP435, D44, D60 and NP205 divorced transfer case. Magnum 5.2 MPI coming soon.